Papermaking involves the forming, pressing and drying of cellulosic fiber sheets. The forming process includes the step of depositing an aqueous stock solution of the fibers, and possibly other additives, onto the forming fabric upon which the initial paper web is formed. The forming fabric may run on a so-called Gap Former machine in which the aqueous stock initially is de-watered, and the initial paper sheet is formed between two forming fabrics.
An effective forming process typically produces a sheet with a very regular distribution of fibers and with a relatively high solids content, i.e., a high fiber-to-water weight ratio. In order to form a fibrous web with a desired uniform, regular distribution and high fiber-to-water weight ratio, the forming fabric must possess a number of properties. First, the papermaking surface should be relatively planar; resulting from the yarn floats in both the machine direction (MD) and cross-machine-direction (CD) lying at substantially the same height, to thereby prevent localized penetration of the fibers into the fabric. Such localized penetration results in “wire marks,” which actually are the result of basis weight variations throughout the sheet area. In addition, the MD and CD floats need to be distributed in a regular manner to avoid introducing undesired wire marks into the formed sheet. Moreover, these basis weight variations can result in undesired variations in sheet absorption properties; a property very relevant to the functionality of quality graphical papers where a consistent uptake of print ink is necessary to produce a clear sharp image.
Other factors also cause the formation of undesired wire marks. For example, wire marks can be introduced into the sheet by the flow of water around yarns positioned below the fabric's papermaking surface. This phenomena, referred to as “strike through,” needs to be taken into account in designing the fabric construction.
Importantly, the forming fabric must also possess a high degree of dimensional stability. This high stability is necessary, for example, to minimize cyclic variations in fabric width, which can result in MD wrinkles in the fabric. This, in turn, contributes to the so-called, streaky sheet, i.e., a sheet with machine direction streaks created by variations in fiber basis weight.
Dimensional stability of a fabric typically is obtained by manufacturing the forming fabric with a relatively high mass of material. However, the use of thick yarns to establish such a high mass often causes undesirable wire marks. Consequently, there has been a trend to providing composite forming fabrics, that is, “multi-layer” structures, whereby a high number of relatively thin yarns are distributed throughout various fabric layers to enhance fabric stability.
One type of multi-layer structure is the so-called triple-layer, or composite, fabric made by joining two (2) distinct fabrics, each with its own MD (warp) yarns and CD (weft) yarns, by the use of additional and independent “binding yarns.” These binding yarns can be employed in either the MD or CD directions, and in this system provide the sole function of binding the two separate fabrics together. In other words, these binding yarns are not intended to function as part of the warp or weft yarn system in either the top fabric or the bottom fabric of the multi-layer structure. Such a triple-layer fabric is illustrated in EP 0,269,070 (JWI Ltd.), the entire subject matter of which is incorporated herein by reference.
Where the two fabrics of the triple-layer structure are joined in either the machine direction or cross-machine-direction by binding yarns that also belong, or form part of the weave pattern of either, or both, of the paper side or wear side fabrics, the resulting structures are referred to more specifically as “self-stitched” triple-layer structures. Such binding yarns are referred to as intrinsic binding yarns. Self-stitched structures are taught in a number of prior art patents. For example, U.S. Pat. No. 4,501,303 (Nordiskafilt AB) discloses a triple-layer structure wherein paper side yarns are used to bind the paper side and wear side fabrics into one structure. The entire subject matter of this latter patent is incorporated herein, by reference.
Triple-layer structures, whether employing separate and distinct binding yarns or intrinsic binding yarns that form part of either the paper side or wear side weave structure, allow, to some extent, for the use of fine MD and CD yarns in the top, paper side fabric for improved papermaking quality and sheet release. Additionally, the use of significantly coarser yarns can be employed in the bottom, lower fabric, or wear side fabric, which contacts the paper machine elements, to thereby provide good stability and fabric life. Thus, these triple-layer structures have the capability of providing optimum papermaking properties in the paper side fabric and optimum strength properties in the wear side fabric.
In the triple-layer and self-stitched fabrics of the prior art the internal surface of the wear side fabric is dominated by floats of machine direction yarns. Where wear side fabric CD yarns interlace with wear side fabric MD yarns, such that the wear side CD yarns appear in the internal region between the paper side and wear side fabric layers, relatively prominent short weft knuckles are formed. The pressure of relatively stiff wear side MD yarns acting on the wear side CD yarns during the production of the fabric can cause so-called “knuckle spread,” whereby the wear side CD yarn knuckles are distorted and their width increased to form a relatively large area. The location of such yarn mass areas within the fabric inner region reduces the ability of water to flow through the fabric in such yarn mass areas such that fabric dewatering may be adversely effected.
A further common feature of the known self-stitched and other triple-layer designs is that they are relatively thick structures with a high amount of void space distributed throughout their thickness. The relatively high “void volume” is typically associated with sheet re-wetting on the paper machine such that the sheet solids content at transfer to the press section may be undesirably low. That is, the fibrous web formed on the papermaking fabric has an undesirably low fiber-to-weight ratio. This can result in reduced machine performance through a higher amount of sheet breaks occasioned by the wetter sheet, reduced running speed and higher drying costs downstream of initial web formation on the papermaking fabric.
A variety of composite fabrics employing intrinsic interchanging yarn pairs have been disclosed to attempt to deal with the various problems of fabric stability e.g., fabric stiffness, desired papermaking side performance and desired wear side performance. In particular, various different composite fabric constructions are disclosed in U.S. Pat. No. 5,826,627 (Seabrook, et al.); U.S. Pat. No. 5,967,195 (Ward); U.S. Pat. No. 6,145,550 (Ward), and International Publication WO 02/14601 A1 (Andreas Kufferath GMBH & Co. KG). The entire subject matter of all these latter-identified patents and publications is incorporated herein, by reference.
In the above mentioned prior art composite fabrics employing intrinsic interchanging weft yarn pairs, each yarn of the pair forms part of the paperside weave pattern and, at least one yarn of the pair, also functions to bind the two fabric layers together. The two members of each pair of interchanging yarn pairs between them form a continuous weft path in the fabric paper side layer. Interchange, or transition, points occur where one yarn of the pair leaves the paperside surface, to bind on the lower fabric layer, and where the other yarn of the pair enters the paperside surface to continue the weave pattern initiated by the first member of the yarn pair. As disclosed in the previously identified Ward '195 and '550 patents, at each transition point the warp yarn around which the pair members transition is disturbed such that an irregularity occurs in the paperside surface. The disturbance can contribute to the formation of undesired sheet wire marks. In the prior art fabrics, on average, a paperside transition point occurs once in every four, five, or six warp yarns. In other words, between 25% and 16.7% of the paperside warp yarns interlacing with any interchanging yarn pair are transitional warp yarns with an inherent tendency to mark the sheet.
Furthermore, the weave patterns employed in the wear side layers of the above-mentioned prior art fabrics do not provide the desired wear resistance for enhanced fabric life. Specifically, these prior art wear side fabric weave patterns have been relatively small, e.g., five or six shaft repeats, such that fabric life potential may be restricted. Moreover, these small shaft repeats create an undesired high frequency of wear side weft knuckles located in the fabric interior, which interferes with the flow of water through the fabric.
Troughton U.S. Pat. No. 6,244,306 has more recently disclosed a self-stitched fabric including a wear side layer with either an eight or a ten shaft fabric repeat pattern. However, the wear side layer weaves disclosed in the Troughton '306 patent utilize multiple warp interlacings with each wearside weft yarn such that there is still an undesirably high amount of wearside weft knuckle material appearing in the fabric interior. Furthermore, the fabrics disclosed in the Troughton '306 patent all have a high frequency of paperside transition points (described in detail hereinafter) and so do not resolve the problem of wire marks stemming from the transitional regions.
For all embodiments of the above prior art structures there is typically a wearside weft passed above a respective wear side warp, on average, once in every four, five, or six adjacent warp yarns. In other words, for each wearside weft, between 25 and 16.7% of its interaction with the wearside warp yarns occurs with the wearside weft inside the fabric, thus restricting the wearside weft material available to provide wear resistant properties for the fabric. In addition, this interaction of the wear side warp yarns with the wear side weft yarns in the inside of the fabric creates a high tendency to interfere with, and create non-uniformity of water flow through the fabric. This can result in irregularities in the formed sheet.
Although the aforementioned composite papermaking fabrics employing intrinsic interchanging yarn pairs have provided improved structures, applicant believes that there still is a need for additional, improved composite structures of the type employing intrinsic interchanging yarn pairs, providing reduced transitional region marking of the paper sheet and reduced occurrences of wearside weft material within the fabric internal region to thereby reduce interference of water flow through the fabric and to increase weft material available for wear. It is to such structures that the present invention is directed.